Hydrocyclones

ABSTRACT

A hydrocyclone has a generally conical chamber, a tangential inlet at the large end of said chamber and an axially aligned accept outlet located centrally of said large end. A reject passage is located at the apex end of said chamber the inwardly facing surface of said reject passage being provided with means defining a spiral shaped groove along and within which portions of the reject material may travel during passage through the reject outlet.

United States Patent Reid et al. Apr. 2, 1974 HYDROCYCLONES 2,222,930 11 1940 Arnold 183/83 4 .1 [75] Inventors: Charles M. Reid; Bernard M. 2 5/1970 Come 209,144

Rmellenmm both Calgary FOREIGN PATENTS OR APPLICATIONS Alberta, Canada 910,797 11/1962 Great Britain [73] Assignee: Elast-O-Car Products & Engineering Limited, Calgary, Alberta, Canada Primary Examiner-John Adee [22] filed Sept 1971 Assistant Examiner-Robert H. Spitzer [21] Ap No; 184,055 Attorney, Agent, or FirmSpencer & Kaye [30] Foreign Application Priority Data Aug. 20, 1971 Canada 121,004 [57] ABSTRACT Sept. 28,1970 Canada 94,215 A hydrocyclone has a generally conical chamber, a tangential inlet at the large end of said chamber and [52] [1.8. CI 210/84, 55/191, 209/144, an axially aligned accept outlet located centrally of 210/512 said large end. A reject passage is located at the apex [51] Int. Cl B0111 21/20 end of Said chamber the inwardly facing Surface of [58] held of Search 55/191 said reject passage being provided with means defining 209/144 210/84 537 a spiral shaped groove along and within which portions of the reject material may travel during passage [56] References C'ted through the reject outlet.

UNITED STATES PATENTS 3,399,770 9/1968 Salomon 209/211 1 Claim, 9 Drawing Figures PATENTEDAPR 21914 $800,946

sum 1 or 5 PATENTEUAPR 2 I974 380.0 946 sum 2 or 5 PATENTEU R 2 I974 SHEET 3 [1F 5 PER CENT EFFICIENCY FIG. 7

PATENTEBAPR 2 L974 PER CENT REJECT RATE N o: h 01 m \1 00 no SHEET UF 5 REJECT CONSISTENCY (/o A. D.)

FIG. 8

HYDROCYCLONES BACKGROUND OF THE INVENTION The present invention relates to the fractionating of slurries or suspensions by means of hydrocyclones.

Hydrocyclones have been in use for a number of years in various fields, for example, the pulp and paper industry and have been found useful for removing certain impurities or forms of dirt of a character unsuited for removal from the pulp by screening. Examples of such impurities are shives bark, grit and some resinous materials.

The general design and manner of operation of a hydrocyclone separator is well known. Briefly, each hydrocyclone includes a body having an elongated conical chamber of circular cross section. An outlet (termed a reject outlet) for the reject or heavy fraction is provided at the apex of the conical chamber. The lighter or accept fraction of the suspension exits through an axially arranged vortex finder and accept outlet at the opposite end of the conical chamber in the center of the latter. The pulp suspension is introduced into the conical chamber via one or more tangentially directed inlets adjacent the large end of the conical chamber, the liquid simultaneously spiralling inward and downward at increasing velocity. An upflowing helical stream having a maximum diameter approximately equal to that of the accept outlet develops about the central axis and surrounds what is commonly termed an air core. The centrifugal forces involved throw the heavier particles in the suspension outwardly toward the wall of the conical chamber thus causing a concentration of solids adjacent thereto while the lighter particles are brought toward the center of the chamber and are carried upwardly by said upflowing helical stream and outwardly through the accept outlet. The heavier particles are caused to spiral downwardly along the interior wall of the hydrocyclone and eventually pass outwardly of the reject" outlet.

The velocities of the fluid within the hydrocyclones under consideration are quite high and the dynamic forces thus produced are so high that gravitational forces have a negligible effect on the performance of the device. As a result, the hydrocyclones under consideration may be oriented in various ways i.e., horizontally, vertically or obliquely while maintaining satisfactory performance.

The hydrocyclones may be arranged in large banks of several dozen or even several hundred hydrocyclones with common feed, accept, and reject chambers suitably communicating with the hydrocyclones.

Many designs of hydrocyclone have evolved in the past in an attempt to maintain the percentage of marketable pulp or light fraction in the rejected fraction, as low as possible while keeping the separation efficiency as high as possible. The bulk of these designs have centered around the apex or reject outlet and all of them were designed to reduce the loss of pulp while at the same time attempting to avoid plugging of the reject outlet.

Plugging of the reject outlets also represented a serious problem in hydrocyclones and was responsible, in large measure, for serious production losses. Older designs had incorporated special reject nozzle flushing or rinsing devices; these involved the use of expensive auxiliary equipment and could not be used in many instances e.g., ahead of a paper machine, where no fluctuations in the flow of suspension were permitted. A more recent development has been the use of the socalled pressurized reject system i.e., a system wherein the rejects are discharged to a pressurized region, which permits the use of larger reject outlets and thus reduces reject outlet plugging somewhat. However, reject outlet plugging was and still is a problem and improvements in this area are indicated.

THE INVENTION Accordingly, it is an object of the present invention to provide an improved method for separating or fractionating liquid suspensions, in a hydrocyclone which method reducers reject outlet plugging, reduces losses of the light fraction of the suspension, and keeps separation efficiency and overall throughput or production at high levels.

In accordance with the present invention, there is provided an improved method of separating or fractionating liquid suspensions in a hydrocyclone having a body defining an enclosed chamber shaped such that it decreases in cross-sectional size from its large end down to its smaller apex end, with a reject outlet por tion at the smaller apex end of the chamber for releasing a heavier fraction of the suspension from the chamber, an accept outlet located generally axially of the chamber at the larger end thereof for releasing a lighter fraction of the suspension from the chamber, and a tangential inlet adjacent the larger end of the chamber.

The method comprising introducing the suspension under sufficient pressure through the tangential inlet into the interior of the chamber so as to produce a fluid vortex within the chamber which surrounds a gaseous core extending along the longitudinal axis of said chamber. The fluid vortex causes heavier fractions of the suspension to be forced outwardly against the wall of the chamber and to be thereafter passed toward and through the reject outlet portion with the lighter fractions of the suspension remaining inwardly of the heavier fractions and being thereafter passed along the axially extending gaseous core, and through the accept outlet.

Portions of the heavier fraction, such as heavy particles and the like, travel along a spiral groove provided by a screw-thread-like formation having a plurality of spaced convolutions which spiral around the axis and are defined in or on the inwardly facing surface of the reject outlet portion. This formation, as seen in a crosssection view taken along the axis, includes a spaced pair of side walls both of which are directed inwardly toward the axis and an inwardly facing top wall extending between those portions of the side walls which are nearest the axis, with the groove being defined between the side walls of adjacent spaced convolutions of the formation. The formation and the groove defined thereby extend to the extreme end of the reject outlet, and the portions of the heavier fraction continually travel along and within the spiral groove and escape from it at the extreme end of the reject outlet regardless of diameter fluctuations of the gaseous core which might otherwise interfere with movement of the heavier portions and tend to momentarily block the reject outlet portion.

Further aspects of the invention are set forth in the claim appended hereto.

The invention will be better understood after reference is had to the following detailed description and to the drawings wherein:

FIG. 1 is a longitudinal cross-section view of typical hydrocyclones according to the invention as they appear when installed in a hydrocyclone separator;

FIGS. 2 and 3 are transverse cross-sectional views taken along lines 22 and 33 respectively in FIG. 1;

FIG. 4 is a longitudinal section view illustrating in detail the configuration of the hydrocyclone reject outlet;

FIG. 5 is a fragmentary sectional view illustrating in detail a portion of the configuration shown in FIG. 4.

FIG. 6 is a view similar to FIG. 1 for the purpose of giving specific dimensional details of a typical hydrocyclone construction;

FIGS. 7, 8 and 9 are graphs illustrating the performance of the hydrocyclone.

DETAILED DESCRIPTION In FIG. 1 there are shown typical hydrocyclones 10 according to the invention disposed in a hydrocyclone separator, only a small portion of which is shown. The hydrocyclones are shown mounted in spaced apart walls 12, 14 and 16, the latter serving to define a discharge or reject region 18, a feed or inlet region and an accept region 22. Suitable seals, e.g., labyrinth seals 21, prevent leakage of material from one region to the other. Each hydrocyclone 10 includes a hollow body portion 24 formed of a suitable wear and abrasion resistant material. Polyurethane elastomers suitable for this purpose have been developed and are preferred.

The interior of the hollow body 24 is of generally elongated conical configuration having a circular cross section, the apex of which has a reject outlet passage 26 extending axially therethrough while the opposite end or base 28 is provided with an axially extending accept outlet 30 the latter including a vortex finder 32 which extends axially part way into the interior of the hydrocyclone body for reasons well known in the art. Adjacent the base 28 of the hydrocyclone there is provided a tangential feed inlet 34 which directs a liquid suspension into the hydrocyclone interior tangentially to the interior wall of the latter. As seen in FIG. 1, the accept outlet 30 of each hydrocyclone communicates with the accept region 22; their feed inlets 34 all communicate with feed region 20, while their reject outlets 26 all communicate with the reject region 18.

In practice, the fluid pressure in feed region 20 is the highest of all e.g., 24-26 psig, the reject region 18 typically has a much lower pressure e.g., 10 or ll psig while the accept region 22 has a pressure which is lower still e.g., 9 or 10 psig.

The overall operation of the hydrocyclone is well known. The liquid suspension enters the tangential feed inlet 34 under pressure and thus sets up a fluid vortex 36 in the interior of the hydrocyclone. The resulting centrifugal forces throw the heavier particles in the suspension outwardly against the interior wall 23 of the hydrocyclone while the lighter fractions remain more closely toward the central axis of the latter, along which the previously mentioned air core extends, such lighter fractions being brought along such central axis and passing outwardly through the accept outlet 30 (often termed the overflow outlet). The heavier fraction travels. in the opposite direction and passes through the reject outlet 26 (often termed the underflow outlet).

In accordance with the invention, the interior wall of the reject outlet passage 26 is provided with spiral screw-thread-like formations 42 which define spiral shaped grooves therebetween and which extendin a plurality of convolutions about the axis of the hydrocyclone. A two-start arrangement is shown in FIG. 4 i.e., two thread-like spirals apart, each with about a one inch lead. As seen in FIGS. 4 and 5 the individual spiral thread-like formations are of a generally rectangular cross sectional shape with the spacing between these formations (which determines the groove width) being roughly equal to their width.

Each screw thread-like formation as shown includes a spaced pair of side walls 42a, 42b which are directed inwardly toward the center of the hydrocyclone chamber and an inwardly facing top wall 42c extending between those portions of side walls 42a, 42b which are nearest the coneaxis. Groove 42 is, of course, defined between the side walls 42a, 42b of adjacent screw thread-like formations. The reject outlet passage 26, which appears as an extension of the interior wall 23 of the hydrocyclone, tapers down in diameter towards the tip of the apex and the passage diameter at the extreme tip may, for example, be about one-half that at the entrance to the reject passage. The embodiment shown has the screw threads spiralling in the right hand direction (looking toward the apex of the hydrocyclone as seen in FIG. 3); that is, the threads spiral toward the apex in a direction opposite to the direction of rotation of the fluid vortex 36 set up within the hydrocyclone chamber. However, it should be realized that in other embodiments the screw threads can twist in the opposite sense i.e., in the same direction as the direction of rotation of the fluid vortex.

The structure of one embodiment of the invention has been described above in general terms. An attempt will now be made to describe why the spiral grooves provided in accordance with the invention improve the performance of the hydrocyclone. It should be noted here that the theoretical considerations involving hydrocyclone performance are rather complex and, because of the confined nature of the liquid within the hydrocyclone, observations and measurements are made very difficult. As a result the following theoretical discussion should not be taken in a strictly limiting sense.

In order to understand why the invention performs as it does, it is necessary to realize that, in operation, hydrocyclones of the type under consideration develop, when operating efficiently, what is known as an air core. In other words, the high velocity rotation of the fluid creates a low pressure axial core having a free liquid surface. The core in a hydrocyclone which communicates directly with the atmosphere at either one outlet or the other becomes air filled. Even if there is no communication between this axial core and the atmosphere, the core still exists filled with vapour and gases that come out of solution in the liquid. In all suspensions and in particular fiber suspensions, gases are always present both dissolved in the liquid and as gas bubbles. The gas bubbles can be removed from the liquid by special treatments but the dissolved gases come out of solution in the vortex of the hydrocyclone and fill the core.

The surface of the air core will always be found to be irregular due to the continuous disturbance from progressive waves thereon analogous to the behaviour which occurs in convergent nozzles. This phenomena is described in a paper by A.M. Binnie, Proceedings of the Royal Society A 205,530 (1951) to which the interested reader may refer for further information.

The air core can show other irregularities. Generally, it is of constant diameter through its length but the diameter increases with increasing flow rate up to a point where further increase has no apparent effect. When rotational velocity adjacent to the air core is impeded however, this ceases to be true and the air core diameter diminishes or the core may even collapse. This may occur within the vortex finder or when solids accumulate in the reject outlet passage.

In any particular hydrocyclone a certain minimum fluid flow rate is required to establish a full air core (i.e., an air core extending entirely through the hydrocyclone including the vortex finder and the reject outlet passage) and additional increase in flow rate causes the air core to expand. Expansion of the air core can in some cases be so great that the air core occupies the whole of the reject outlet passage thus preventing flow of the rejected fraction therethrough.

Reference will now be made to the volume split which may be mathematically defined as the volume rate of the reject flow divided by the volume rate of the accept flow. This volume split can be altered by applying back pressure to the liquid stream issuing from the hydrocyclone outlets. It will be appreciated that the air core diameter is dependent on the hydrocyclone accept pressure condition and, as mentioned above, such diameter increases with an increase in flow rate. This means that the flow rate also affects the volume split and the manner in which it affects it is quite complicated. For a small hydrocyclone, the air core has a greater influence on the volume split than has the air core on a large hydrocyclone.

As the air core in the hydrocyclone developes and increases in diameter there comes a stage where it obscures or occupies a major portion of the exit area or passage and thus any further increase in liquid flow rate decreases the rate of liquid flow through the reject outlet. Ultimately, flow through the reject can cease altogether when the air core diameter exceeds the diameter of the reject outlet. This latter condition is readily demonstrated with water flow and is less readily demonstrated with the flow of the solid-liquid slurry. The reason for this is that in handling the latter, the solids are being rejected towards the reject outlet and accumulation of same in the reject outlet passage even under starved reject flow conditions will cause a decrease in rotational velocity, collapse of the air core, and subsequent discharge through the reject outlet.

Fiber suspensions behave very much like water because of the relatively low concentration of solids. The fibers'are themselves lighter than water. Thus the underflow can be starved to a greater degree without collapse of the air core.

It will be appreciated that the discharge of solids from the hydrocyclone occurs through the annular region existing between the air core and the walls of the reject outlet passage. This phenomena has been observed by various persons skilled in the art of hydrocyclone design and reference is made thereto in Canadian Pat. No. 688,415 to Tomlinson, page 9, lines 3 15 to which the interested reader may refer for further information.

With the above background information in mind, reference will be again made to the specific embodiment shown in the drawings. As readily seen in FIG. 4, the helical groove in the reject passage 26 is defined by a relatively course double screw thread 42 with the top wall 42c of the threads (that portion nearest the hydrocyclone axis) defining an imaginary surface that is generally conical with, preferably, but not necessarily, the same included angle as the section of the hydrocyclone chamber above the threaded section. From observations, it appears that the air core of the hydrocyclone developes in this conical region defined by the imaginary surface referred to above. Firstly, let us consider a hydrocyclone without the screw threaded portion i.e., a smooth walled reject outlet passage. Due to the continuous disturbances and progressive waves in the air core surface as mentioned above (A. M. Binnies paper) there will be corresponding fluctuations in the size of the annulus between the air core and the walls of the reject outlet. It has been shown that at low flow rates a smoke ring effect is obtained as the liquid flow is intermittently cut off by the air core (reference The Hydrocyclone, International Series of Monographs in Chemical Engineering, Volume 4 by B. Bradley, Pergamon Press, see illustration facing page 126). The smoke ring formation is presumably due to the instability of the air core diameter which allows periodic discharge of solids through the annulus between the air core and the reject passage walls. This effect will still exist through a lesser degree at high flow rates but is not so noticable due to the shorter length of the waves (reference Binnie FIG. I, Page 532).

With the above background information in mind, it appears that the spiral grooves provided in the reject passage in accordance with the present invention provide what could be termed an escape route for the heavier solids at all times regardless of the relative diameter of the air core. Hence, these heavier solids will always be able to escape from the hydrocyclone even after great fluctuations in the size of the air core which can be caused by fluctuations in the feed, temporary partial plugging of the feed inlet by over-size particles and other practical considerations such as unscheduled plant shut downs due to power failure etc.

In operation, the hydrocyclone of the present invention functions in such a way that as the solids are flung by centrifugal force against the interior wall of the conical chamber and move downwardly, the spiral grooves between the screw threads 42 provide an escape route at all times and at the smallest diameter of the reject passage 26 (at the extreme apex end of the cone), there will always be room for some solids to be rejected by way of the grooves between the threads even if the central region of the reject passageway is instantaneously or momentarily "closed" by the air core. At the same time the material discharged from the grooves will contain the unwanted dirt, bark, etc. that was removed from the fiber. Thus the reject flow never ceases and the plugging tendency is greatly reduced. This latter observation is born out by tests of the hydrocyclone carried out in a large pulp mill. The records indicate a substantial reduction in plugging tendency when the hydrocyclones of the present invention are used.

The spiral grooves provided in accordance with the present invention also influence the reject rate (which is the percentage ratio of the dry weight of the pulp in the reject fraction to the dry weight of pulp in the infeed). From all available sources including practical experience, it is apparentthat for controlled cleaning of suspensions, for example fiber suspensions, air core stability in the reject outlet passageway is necessary for reasonable control of the volume split and consequently the reject rate. The spiral grooves provided in the hydrocyclone of the present invention permit solids to find their way out under pressure without interfering with the air,core, which solid particles, in a smooth walled arrangement, could otherwise impede rotational velocity adjacent the air core and cause the air core to break down thus causing sudden loss of acceptable slurry through the reject outlet.

The hydrocyclone according to the present invention including spiral grooves in the reject outlet is capable of operating at higher capacities compared with a hydrocyclone of the same dimensions (except for a larger feed inlet opening in the hydrocyclone of the invention) which is operating under the same conditions and which does not include such spiral grooves. With the higher fluid flow rates within the hydrocyclone of the invention, the air core diameter will be larger and the annulus existing between the air core and the imaginary conical surface defined by the top wall 42c or inwardly facing surface of the screw threads at the smallest diameter of the reject outlet passage will be reduced in thickness, but since the grooves defined by the thread are present, the problem of increasing the risk of intermittent loss of flow which could cause either blockage or inefficient smoke ring discharge is absent due to the escape route provided by such grooves as described previously.

Test results suggest an increase in capacity utilizing the hydrocyclone according to the present invention over a conventional hydrocyclone of the same overall size operating under the same conditions. Equivalent cleaning efficiency and a lower reject rate are achieved at the same time by the hydrocyclone of the invention.

Another factor to be considered is the percentage of the flow through the reject passage. This is commonly termed the percent of flow to reject and is in effect the percentage of total infeed that is discharged through the reject outlet. The higher the temperature of pulp in a pulp mill, the greater will be the percent of flow to reject. Woodruff, in Canadian Pat. No. 821,918 states that it has been shown that a standard separator has been adversely affected by increase in pulp temperature, the rejection rate more than tripling under comparatively modest temperature rises as from 90F to l l 1F.

Modern bleach plants discharge their pulp at even higher temperatures e.g., 120F. 130F. Woodruff reporting in Pulp and Paper, March 25, 1968 states that the viscosity of water changes appreciably with temperature... As the viscosity of water decreases its hydraulic drag decreases thereby diminishing the force that carries the pulp fiber towards the upward vortex and out with the accepted fraction. This results in more fibers remaining in the proximity of the cleaner wall and ultimately leaving with the reject fraction. Later on he also stated that if the reject nozzle were made smaller in diameter to reduce the reject rate the pulp dewatered so rapidly that it plugged the reject nozzle".

Hydrocyclones in accordance with the present invention have been used very successfully in treating pulp at higher temperatures e.g., between and F. under high feed consistencies of between 0.55 and 0.65. Under the above conditions, conventional hydrocyclones experienced severe plugging problems. The use of a spiral groove in hydrocyclones in accordance with the present invention reduce the number of cones plugged in one weeks operation at a pulp mill in a unit employing 400 hydrocyclones from between five percent or more to one percent under the identical conditions.

A somewhat theoretical discussion has been given above explaining how and why the spiral grooves provided in the hydrocyclone in accordance with the invention function and the advantages associated with same. We will now make reference to certain desirable structural features.

It will be seen from FIGS. 1 and 4, etc. that the spiral grooved portion extends from the extreme apex end of the hydrocyclone towards the opposite end for a certain distance. By observation and experiment, it has been found that the threaded portion should extend from the apex end to a position above or beyond the socalled plane of no return for best results. This plane of no return has been defined as that plane below which the solids portion flows to the reject outlet without reversing its path (refer to British Pat. No. 799,394 Dahlstrom, Jan. 24, 1955). This plane of no return is also assumed to be the axial position at which the inner vortex (i.e., the upwardly flowing swirling stream surrounding the air core) commences. Salomon in Canadian Pat. 839,550 refers to this plane of no return as aicritical depth. I w Some mention has already been made of the design for the threads 42 forming the spiral grooves. Firstly, the imaginary surface formed by the top wall 42c (inwardly facing surface) of the threads should be conical. The thread should preferably, but not necessarily, be a multiple thread, with the edges of the thread, as seen in cross-section, being slightly rounded. Both the lead and pitch of the thread should be chosen so as to control the ratio of the surface area of the top wall 420 of the thread to the total area of the threaded section to the proportion required depending on operating variables. The thread groove need not be of constant depth and its profile can change along its length. The profile of the thread groove can vary greatly e.g., can be semicircular, semi-elliptical, rectangular and so on. In general, each design depends somewhat on operating variables such as feed consistency, temperature, type of suspension, capacity, etc.

It is also desirable to make mention of the thread direction i.e., whether it is in a positive or negative sense relative to the direction of the fluid vortex within the hydrocyclone. FIGS. 2 and 3 illustrate threads cut in a negative direction. As seen in FIG. 2, the fluid vortex 36 (designated by the arrow) rotates in a counterclockwise direction; as seen in FIG. 3, the screw threads are in such a direction that the reject material passing along the grooves defined thereby rotates in a clock-.

wise direction. This type of arrangement is known as a negative thread". The opposite arrangement i.e.

where the threads are in a direction such that the reject 1 lngjh f mice, Dunc, passage fa material passing along the grooves rotates in the same D "f 'f of Cyclone Chamber 1 /16 inch k E diameter of re ect outlet passage at apex A inch direction as the fluid vortex is nown as a positive F diameter ofmjccl outlet passage in its thread arrangement. G lcmfr nc H r h b i inch an 6 O II'IICIIOI' W3 0 C alTl El 0 h cholce. of q y OI: negative thread f 5 g long axis of hydrocyclone (this angle ments depends somewhat upon the feed consistency; ng 21 a 1% s 9 i a. h t th t b i ht f l in l reject outlet passage) 4% w I? represen S e percen age we 0 pu I F, length of vortex finder [9% inch the infeed slurry. For low feed consistencies a negative d,min. internal diameter of vortex finder 9/16 inch F length of cylindrical portion of chamber 2 inch thread is employed to reduce plugging thus making Timid details (FIG 5) (Two lhrcadmke lower re ect rates possible without loss of cleaning effil0 formations me apart, I inch lead) ciency. The flow of dirt, etc. in the grooves defined by g v n r I the thread is effectively in the reversed direction, oppoinch site to that of the rotating liquid radially inwardly of the J v. inch K V: ineh groove, thereby producing sufficient turbulence between the two flows to reduce matting of the fiber and I5 plugging of the grooves etc. of the reject outlet passage. With higher feed consistencies at higher or equal oper- In Example I the helical thread in the negative direcating temperatures it appears that the higher turbution i.e. oppositely directed to direction of feed inlet. lence developed must decrease air core stability result- The hydrocyclone was supplied with wood pulp slurry ing in higher reject rates and lower cleaning efficienat 90 100 F. Specific data is given in Table I which eies. In the latter case, it has been found desirable to f llo TABLE 1 T (Wood Pulp Slurry Feed temperature 90 I00F).

Test Capacity Pressures (PSIG) Consistency AD Reject Dirt Count/(grams) Per Cent No. (Total throughput) Feed Accepts Rejects Feed Reject Rate Feed Accept Efficiency Oulet Outlet 1. I05 U.S. gals/min. 24 9 ll 0.280 0.428 4.0% 255 33.0 86.0% 2. do. 24 9 l0 0.295 0.320 10.0% 234 49.l 79.0% 3. do. 24 9 10 0.265 0.301 9.96% l5l 28.7 81.0% 4. do. 24 9 l0 0.305 8.80% I85 407 78.2% 5. do. 25 I0 I l 0.3 l0 0.496 7.30% I50 40.0 74.0% 6. do. 25 I0 ll 0.300 0.410 5.00% I73 50.2 70.5% 7. do. 26 9 10 0.280 0.428 4.00% 195 62.4 67.5% 8. do. 26 9 10 0.270 0.5I6 3.50% 2H 79.0 62.0% 9. do. 26 9 10 0.290 0.573 6.00% 205 57.4 72.0%

scribed in general terms. Specific examples of hydrocyclones according to the invention will now be given along with tables giving the results of tests carried out on same.

EXAMPLE I The specific dimensions of the hydrocyclone whose test results are given in Table l and plotted in FIGS. 7

and 8 are given below, reference being had to FIGS. 5 and 6.

Dimensions k A cross-sectional area of tangential inlet 0.]93 sq. inch B overall length of hydrocyclone chamber and reject outlet passage 13% inch In connection with Table I it is noted that the "reject consistency is the percent ratio of the dry pulp contained in the rejects fraction e.g., 0.428 percent consistency means 0.428 percent of the rejected fraction is pulp. Feed consistency indicates the percent of pulp in the infeed in like manner. The Reject Rate is the percent ratio of the dry weight of the pulp in the reject fraction to the dry weight of pulp in the infeed. The percent Efficiency represents the percent of dirt removed from the incoming feed as given by the feed and accept dirt counts.

The results t ab ulated in Table I are plotted in FIG S.

7 and 8. The curve given in FIG. 8 is of particular importance. This curve shows that a reject rate as low as 3.5 percent can be achieved with the very low reject consistency of about 0.48 percent with very little evidence of reject outlet plugging. This compares very favorably with a prior art hydrocyclone of the same capacity tested under the same conditions which achieved a minimum reject rate of 6.5 percent at a reject consistency of about 0.84 percent with a substantial degree of reject outlet plugging problems. This prior art hydrocyclone had the same general dimensions as the one described above but incorporated spaced, parallel, ring-type choking members in the reject outlet passage.

The sharp increase in reject rate as the reject consistency was increased from 0.5 percent through to 0.6

temperatures (90 The following (Table 111) gives the results obtained when the hydrocyclone of Example I (including negative screw thread) was tested under conditions the same as those of Table II.

Table 111 (Wood Pulp Slurry Feed Temperature 120 130F) Test Capacity Pressure Feed Con- Reject Cleaning No (Total Through A P (psi) sisteney of Rate Efficiency X:

put)U.S. gals/min. dry pulp infeed) passage is increased. Beyond a certain point however, W The variations in reject rate in the above Tables is destruction of the air core in the reject passage may take place, causing a sudden increase in the amount of good pulp rejected. This is what apparently happened in the test plotted in FIG. 8.

EXAMPLE II The specific dimensions of this hydrocyclone are the same as those of Exarrrple 1. However, tlg h lic tl thread 42 was arranged in the positive sense i.e., in the due in large measure to fluctuations in the infeed temperature within the ranges given.

Table IV gives the results of a test carried out on a hydrocyclone having the same overall dimensions as those of Examples 1 and 11 but wherein the spiral grooves were absent and spaced, parallel, ring-type choking members used instead. The increase in hydrocyclone capacity and the lower or equivalent reject rates at about the same feed consistency made possible same direction as the direction of the feed inlet. This by the present invention are apparent f a ihydrocyclone was supplied with wood pulp slurry at a relatively high temperature (120 130F.) and the results of the tests carried out thereon are set out in Table 11 which follows:

Table 11 son of Tables 11 and 1V (compare in particular Test 6, Table 11, to Test 1, Table 1V, both having A p 21 psi). In addition the frequency with which the reject noz- 'zles plugged when using hydrocyclones according to (Wood Pulp Slurry Feed Temperature 120" lflly'l") Test Capacity Pressure Feed Con- Reject Cleaning No. (Total Through- AP (psi) sisteney(% of Rate Efficiency put) U.S. gals/min. dry pulp infeed) A P: difference between feed pressure and accept pressure.

FIG. 9 plots percent Efficiency vs. reject rate for the three different values of A P and feed consistency of Table 11. The following observations may be made in connection with Example 11 and Table 11.

It was noticed from over a month of testing that,

l. a slight variation in feed consistency will affect cleaning efficiency. Higher feed consistencies are employed so as to achieve higher pulp production rates without having to increase the physical capacity of the cleaning system e.g. by using larger pumps, lines, valves, etc.

2. by increasing reject rate, cleaning efficiency increases but at the same time pulp loss also increases.

3. The temperature of the slurry dictates to a great extent, the reject rate. However, higher temperatures are considered desirable by some mills because this lowers dewatering time downstream of the cleaners.

ljgr ce the high reject rates in Example 2.

It is also believed to be appar eht fr om a review of Tapreferable at low feed consistencies and normal infeed 100F), but that the positive thread of Example 11 is preferable under higher feed consistencies and higher (120 130F) infeed temperatures.

the present invention was found to be about 1/5 that experienced when using the above mentioned prior art hydrocyclones.

Although the invention has been described with a certain degree of particularity it is to be understood that many changes can be made within the spirit of the invention. Suspensions or slurries other than the wood pulp suspensions specifically mentioned herein may be fractionated by using the present invention. For example, in certain applications, the heavier material passing We aim;

1. A method of separating or fractionating liquid suspensions in a hydrocyclone having a body defining an enclosed chamber shaped such that it decreases in cross-sectional size from its large end down to its smaller apex end, with a reject outlet portion at the smallerapex end of said chamber for releasing a Table IV (Prior Art Hydrocyclone) (Wood Pulp Slurry Feed Temperature l l F) Test Capacity Pressure Feed Con- Reject Cleaning No (Total Through- A P (psi) sistency of Rate EtTciency put) U.S. gals/min. dry pulp infeed) heavier fraction of the suspension from said chamber, an accept outlet located generally axially of said chamber at the larger end thereof for releasing a lighter fraction of the suspension from said chamber, and a tangential inlet adjacent said larger end of the chamber, the method comprising introducing the suspension under sufficient pressure through said tangential inlet into the interior of said chamber so as to produce a fluid vortex within said chamber which surrounds a gaseous core extending along the longitudinal axis of said chamber said fluid vortex causing heavier fractions of the suspension to be forced outwardly against the wall of said chamber and to be thereafter passed toward and through said reject outlet portion with the lighter fractions of the suspension remaining inwardly of the heavier fractions and being thereafter passed along the axially extending gaseous core, and through said accept outlet, characterized in that portions of said heavier fraction, such as heavy particles and the like, travel along a spiral groove provided by a screw-thread-lilte formation having a plurality of spaced convolutions which spiral around said axis, and defined in or on the inwardly facing surface of the reject outlet portion, said formation, as seen in a cross-section view taken along said axis, including a spaced pair of side walls both of which are directed inwardly toward said axis and an inwardly facing top wall extending between those portions of said side walls which are nearest said axis with said groove being defined between the side walls of adjacent spaced convolutions of said formation, said for- 

